Kwok Hung Li

In this letter, we propose to implement an index modulation (IM) scheme in a reconfigurable intelligent surface (RIS) assisted downlink non-orthogonal multiple access (DL-NOMA) system. Specifically, by allocating different numbers of RIS elements per user during each transmission, we can convey additional information bits. The proposed scheme improves the channel quality while simultaneously increasing the spectral efficiency (SE). We use a simple two-user communication model with PSK modulation to explain the concepts and derive new expressions for the PSK NOMA symbol error rate (SER). Simulations show that as the number of RIS elements increases, the improvement in transmission rate is at the cost of only a small degradation in the error rate performance.

In this letter, we demonstrate that the symbol error rate (SER) performance of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted downlink non-orthogonal multiple access (DL-NOMA) communication system can be improved by jointly setting the NOMA power allocation and the STAR-RIS amplitude parameters. The power allocation parameter mainly determines the relative symbol distances in the NOMA superimposed signal constellation, while the STAR-RIS amplitude parameter can dictate the relative strength of the users’ effective channel. By scaling both parameters together, the overall detection error can be reduced. Using a simplified two-user model with QAM modulation, the proposed schemes are explained, and simulations confirm the improved SER performance versus baseline schemes.

Error-rate performance is one of the important metrics in any communication system. In this letter, we leverage on the ability of the simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) to alter distinctively the channel characteristics of its reflection and transmission zone, enhancing the performance of non-orthogonal multiple access (NOMA). Specifically, to improve the symbol-error rate (SER) performance of an uplink NOMA system when users are employing QAM modulation, we propose to adjust the phase-shift and amplitude parameters of the STAR-RIS based on the distance-maximizing constellation scaling and rotation. The proposed schemes are explained using a simplified model, and simulations show that they significantly improve the SER performance of the system compared to other baseline schemes.

Reconfigurable intelligent surface (RIS) is a cost-effective method to enhance the end-to-end wireless channel. Meanwhile, Non-Orthogonal Multiple Access (NOMA) improves the overall spectral efficiency, connectivity and latency by allowing different users to utilize the same resource blocks. Combining these two technologies has been the main focus of recent research. In this letter, we propose a RIS-assisted NOMA with an interference-cancellation scheme, wherein a group of phase shift elements is assigned to mitigate interference to other users not being supported. We also propose an interference-alignment scheme when RIS is supporting all the NOMA users. In this particular scheme, we align the RIS-amplified signal of each user to the interference for better reception. We analyze the symbol error rate (SER) performance of these schemes in a downlink M -QAM NOMA system. We adopt a simple model where we derive approximate SER expressions and bounds, and simulate them to validate the results.

In this article, we investigate two-tier nonorthogonal multiple access (NOMA)-based wireless powered communication networks. The considered network consists of a hybrid access point and two groups of users, namely near and far users. A near user and a far user are selected to form a NOMA pair, which harvest energy during the downlink phase and transmit information during the uplink phase. Specifically, we consider two cases of energy-transfer time and analyze the corresponding performance. We first analyze the performance of the case with fixed energy-transfer time and derive the exact expression for the ergodic rate. Due to the doubly near-far effect, the far user experiences poor performance. To address this issue, an adaptive energy-transfer time scheme is designed by maximizing the far user’s data rate, and the optimal solution is derived. To reduce the system complexity, a suboptimal solution is proposed, and the ergodic rate's exact expression is derived. Simulation results reveal that the suboptimal solution achieves near-optimal performance and has a lower computational complexity. Moreover, a one-tier NOMA network is set to be a benchmark, and numerical results show that the two-tier system outperforms the one-tier system significantly under the appropriate radius setting.

In this paper, non-orthogonal multiple access (NOMA) networks assisted by multiple intelligent reflecting surfaces (IRSs) with discrete phase shifts are investigated, in which each user device (UD) is served by an IRS to improve the quality of the received signal. Two scenarios are considered according to whether there is a direct link between the base station (BS) and each UD, and the outage performance is analyzed for each of them. Specifically, the asymptotic expressions for the upper and lower bounds of the outage probability in the high signal-to-noise ratio (SNR) regime are derived. Following that, the diversity order is obtained. It is shown that the use of discrete phase shifts does not degrade diversity order. More importantly, simulation results reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance. Simulation results also imply the superiority of IRSs over full-duplex decode-and-forward relays.

In this paper, non-orthogonal multiple access (NOMA) networks assisted by multiple intelligent reflecting surfaces (IRSs) with discrete phase shifts are investigated, in which each user device (UD) is served by an IRS to improve the quality of the received signal. Two scenarios are considered according to whether there is a direct link between the base station (BS) and each UD, and the outage performance is analyzed for each of them. Specifically, the asymptotic expressions for the upper and lower bounds of the outage probability in the high signal-to-noise ratio (SNR) regime are derived. Following that, the diversity order is obtained. It is shown that the use of discrete phase shifts does not degrade diversity order. More importantly, simulation results reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance. Simulation results also imply the superiority of IRSs over full-duplex decode-and-forward relays.

Intelligent reflecting surfaces (IRSs) are envisioned to provide reconfigurable wireless environments for future communication networks. In this paper, both downlink and uplink IRS-aided non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) networks are studied, in which an IRS is deployed to enhance the coverage by assisting a cell-edge user device (UD) to communicate with the base station (BS). To characterize system performance, new channel statistics of the BS-IRS-UD link with Nakagami-$m$ fading are investigated. For each scenario, the closed-form expressions for the outage probability and ergodic rate are derived. To gain further insight, the diversity order and high signal-to-noise ratio (SNR) slope for each scenario are obtained according to asymptotic approximations in the high-SNR regime. It is demonstrated that the diversity order is affected by the number of IRS reflecting elements and Nakagami fading parameters, but the high-SNR slope is not related to these parameters. Simulation results validate our analysis and reveal the superiority of the IRS over the full-duplex decode-and-forward relay.

Intelligent reflecting surfaces (IRSs) are envisioned to provide reconfigurable wireless environments for future communication networks. In this paper, both downlink and uplink IRS-aided non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) networks are studied, in which an IRS is deployed to enhance the coverage by assisting a cell-edge user device (UD) to communicate with the base station (BS). To characterize system performance, new channel statistics of the BS-IRS-UD link with Nakagami- $m$ fading are investigated. For each scenario, the closed-form expressions for the outage probability and ergodic rate are derived. To gain further insight, the diversity order and high signal-to-noise ratio (SNR) slope for each scenario are obtained according to asymptotic approximations in the high-SNR regime. It is demonstrated that the diversity order is affected by the number of IRS reflecting elements and Nakagami fading parameters, but the high-SNR slope is not related to these parameters. Simulation results validate our analysis and reveal the superiority of the IRS over the full-duplex decode-and-forward relay.

In this letter, we propose a two-step userparing scheme for an orthogonal frequency-division multiplexing (OFDM)-based cooperative non-orthogonal multiple-access (NOMA) system where far users cannot communicate with the base station directly. The first step is for allocating near users, and the second step is for allocating far users. To evaluate the performance of the proposed scheme, we derive the closed-form expression for the upper bound of the outage probability, and then obtain the achievable diversity gain. Numerical results show that the proposed scheme can achieve the optimal diversity gain confirmed by exhaustive search.